1. Field of the Invention
The present invention relates to a wavelength calibration technique, and particularly to a wavelength calibration technique wherein a reference beam including one or more lines of known wavelength and an excimer laser emission simultaneously impinge upon a reflection grating and a position sensitive detector, such that an absolute wavelength of the excimer laser beam may be calibrated based on relative positions of the lines at the detector.
2. Discussion of the Related Art
Narrow band excimer lasers have particular applicability in the field of photolithography of integrated circuit (IC) chips. Excimer lasers generally emit radiation at short wavelengths. KrF-and ArF-excimer lasers, e.g., emit in the deep ultraviolet (DUV) region of the electromagnetic spectrum, i.e., .about.350 nm to .about.190 nm, at .about.248.3 nm and .about.193.3 nm, respectively. F.sub.2 -lasers emit in the vacuum ultraviolet (VUV) region, i.e., .about.190 nm to .about.100 nm, at .about.157.6 nm. F.sub.2 -lasers differ from ArF- and KrF-excimer lasers because the F.sub.2 molecule is stable in the ground state, whereas the ArF and KrF molecules may only be found in an excited state. We note, however, that wherever we refer to "excimer" lasers in this application, including the claims, we intend to include the F.sub.2 -laser. KrF-excimer lasers are used for producing quarter and sub-quarter micron structures on IC chips. ArF-excimer lasers are used for producing structures in the range from 0.18 .mu.m to 0.13 .mu.m. F.sub.2 -lasers may be used to form even smaller structures due to their smaller output emission wavelength.
Achromatic imaging optics for the DUV and VUV wavelength ranges are difficult to produce. Very narrow bandwidth excimer laser radiation is thus needed for photolithographic application in order to prevent errors caused by chromatic aberrations. Exemplary acceptable bandwidths for different imaging systems and the KrF- and ArF-excimer laser wavelengths, i.e., 248 nm and 193 nm, respectively, are shown in Table I.
TABLE I ______________________________________ (requirements on radiation bandwidth) imaging optics .apprxeq. 248 nm .apprxeq. 193 nm ______________________________________ refractive optics &lt;0.8 pm 0.3 pm-0.7 pm catadioptics 50 pm-100 pm 20 pm-40 pm ______________________________________
In their free running operation, excimer lasers such as KrF- and ArF-lasers have bandwidths of .about.500 pm. This range of emission wavelengths is known as the broadband emission spectrum of the laser. Thus, band narrowing optics are installed within the cavities of these lasers to narrow their bandwidths to those shown in Table I.
It is also desirable to have complete tunability of the narrowed band emission of the excimer laser throughout its entire broadband emission spectrum. Thus, it is desirable to have the center wavelength of the spectrally narrowed laser emission be tunable within the entire gain curve of the laser, such that the center wavelength may be tuned through, e.g., a 200 pm to 600 pm spectral range. For most applications, it is satisfactory to have tunability in a 200 to 300 pm range around the center of the broadband emission spectrum of the laser.
Accurate and precise wavelength calibration of the center of the narrowed excimer laser emission band and its stability from pulse to pulse over time is very important. Table II shows calibration accuracies and pulse to pulse wavelength stabilities of advantageous excimer laser systems to be used in sub-quarter micron photolithographic processing.
TABLE II ______________________________________ (requirements on wavelength (a) accuracy and (b) stability) = 248 nm = 193 nm imaging optics (a) (b) (a) (b) ______________________________________ refractive optics .+-.0.5 pm .+-.0.05 pm .+-.0.5 pm .+-.0.05 pm catadioptics .+-.10 pm .+-.5 pm .+-.3 pm .+-.1 pm ______________________________________
It is thus clear that the emission wavelength of a useful excimer laser system is measurable with high accuracy. The stability of the emission wavelength of such an excimer laser over operation time is also quite controllable. It is also desired to have a wavelength calibration system wherein the narrowed excimer laser emission wavelength, or the center of gravity of the laser emission spectrum, is compared with a known absolute calibration wavelength.
A wavemeter such as a Fabry-Perot interferometer, where interference fringes are imaged onto a CCD-camera, is normally used to determine the relative wavelength of the laser system. An additional wavelength calibration tool is used for absolute wavelength calibration. Some examples include wavelength calibration systems wherein the narrowed band is tuned through absorption lines of a photoabsorbing gas or vapor. U.S. Pat. No. 5,540,207 to Fomenkov discloses a wavelength calibration technique wherein photoabsorption of laser light by iron (Fe) vapor is used. Transmission intensity is measured as the narrowed emission traverses the Fe-vapor and the wavelength of the narrowed emission is tuned around an absorption wavelength of the Fe-vapor.
U.S. Pat. No. 4,905,243 to Lokai et al. also uses Fe-vapor which has absorption lines around the 248 nm emission wavelength of the KrF-excimer laser. Lokai uses the optogalvanic effect, rather than monitoring a straight-forward photoabsorption, for determining the absolute wavelength of the narrowed emission. Each of the Fomenkov and Lokai procedures are performed by scanning the narrowed emission through a range of wavelengths including at least one photoabsorption line of the gaseous vapor, e.g., Fe. Since the wavelength must be scanned to perform the calibration, the lithography laser system must be taken offline to carry out the procedure. As can be appreciated, no photolithographic processing can be performed as the laser system is being scanned to precalibrate the wavelength. This precalibration procedure results in undesirable system downtime.
Another known technique has been described in U.S. Pat. No. 5,404,366 to Wakabayashi et al., wherein a KrF-excimer laser output beam and a reference beam are directed together through a monitor etalon. An absolute wavelength of the output beam is determined when interference fringes from both the reference beam of known wavelength and the excimer laser output beam are detected alternately, via shuttering, in the focal plane of a condenser mirror. The two beams are not simultaneously detected due to spectral overlap at the mirror surface, yielding a first disadvantage. A second disadvantage of the arrangement of Wakabayashi et al. is that a different etalon is needed for each excimer laser source used. Moreover, a different etalon is needed even when using the same radiation source, when a different bandwidth, e.g., as determined by the optics, is being used. The "free spectral range", or applicable range of wavelengths, of the monitor etalon at any time is simply unsatisfactorily far smaller than a desired tuning range of the narrowed spectral emission. Put another way, the very high interference order (&gt;10.sup.4) of the etalon makes it difficult to clearly determine the absolute wavelength of the excimer laser beam.